X-ray spectroscopy provides a great deal of information for analyzing reactivity and characteristic properties of matter by probing the interactions of molecules with their local chemical environment. The nature of core orbitals and elemental specificity of core-level transitions provide an advantage for resolving spectroscopic signatures with continually advancing experimental X-ray techniques. Computational modeling of X-ray processes can be achieved with ab initio methods for describing the electronic structure of core-excited states. This dissertation explores quantum chemical models for simulating X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and X-ray emission spectroscopy (XES). Specifically, the following research examines the applicability o fΔ-based self-consistent-field (ΔSCF) and Møller-Plesset (MP) perturbation theory (ΔMP) methods and introduces different composite models coupling these techniques with electron propagator theory (EPT) for studying element-specific K-shell transitions.
Firstly, as a practical approach, vertical core excitation energies are obtained using a combination of the ΔSCF method and the diagonal second-order self-energy approximation with the inclusion of relativistic effects. For core excitations involving delocalized symmetry orbitals, the applied method improves upon the overestimation of ΔSCF by providing approximate values close to experimental K-shell transition energies.
Furthermore, spin projected ΔUHF and ΔUMPn (n=2,2.5,3) methods are also used to calculate vertical core excitation energies. These methods are applied to a set of symmetrical molecules with equivalent atoms. The role of core localization, SCF orbital relaxation, pair correlation, and different relativistic corrections on the accuracy and reliability of the results is examined. Additionally, the limitations of using core-hole reference determinants and complications that may arise in perturbative calculations are addressed.
Lastly, a practical ab initio composite method for modeling X-ray absorption and non-resonant X-ray emission is presented. Vertical K-edge excitation and emission energies are obtained from core-electron binding energies calculated with spin-projected ΔHF/ΔMP and outer-core ionization potentials/electron affinities calculated with electron propagator theory. An assessment of the combined methodologies against experiment is performed for a set of small molecules containing second-row elements. Methods for obtaining transition intensities are applied for reconstructing non-resonant X-ray emission spectra.
Results from the various ab initio models examined indicate that sub-electron-volt accuracy can be obtained for core-level energetics while maintaining a satisfactory balance between accuracy and computational cost.
